In many areas of automotive engineering, positions of movable positioning components such as actuating elements are potentiometrically sensed, in particular in connection with engine controls. The movable tap of the potentiometric measuring arrangement is connected to the shaft of the particular positioning component.
The movable potentiometer contact results in abraded material on the slider track of the potentiometer, which forms an insulating layer on the slider track. This may cause misinterpretation of the measuring signal or failure of the measuring arrangement and consequently of the electronic control connected to it. Availability of the system is thus considerably restricted.
Accordingly, the invention is based on the task of specifying measures which lead to an increase in the availability of the engine control system.
In electronic engine power controls, the principle of which is known for example from EP-A 171 641 or from the publication of G. Kolberg: "Elektronische Motorsteuerung fuor Kraftfahrzeuge", Motortechnische Zeitschrift, 46th year, Volume 4, 1985, potentiometers are used for sensing the accelerator pedal position and for position feedback of the throttle flap position. In such systems, the throttle flap is controlled by electronic means directly and/or indirectly as a function of measured variables, among which the accelerator pedal position representing the driver's requirement is counted, by an electronic control unit via a positioning motor connected to the throttle flap and the position feedback potentiometer.
Further, it is known to use such systems in the idle case for controlling the idle speed via the throttle flap. For this purpose, the throttle flap maintains the idle opening angle when the accelerator pedal is released in order to supply the internal combustion engine with the pregiven idle air. To control the idle speed, the throttle flap is then adjusted between its minimum idle position and its maximum idle position.
The insulating abraded material generated by the potentiometer slider on the potentiometer track is pushed back and forth in the driving operation by the movement of the positioning component connected to the potentiometer and becomes deposited on the potentiometer track particularly at the turning points of the movement. There, an insulating layer forms between potentiometer track and slider, which results in the above-mentioned failure of the potentiometer and of the control system.
In the case of an electronic engine power control, the idle position is taken up most frequently, so that the failure frequency of the potentiometer due to dirt accumulation in its idle position is particularly great. For safety reasons, the electronic engine power control reacts to this error, which can be detected by means of a plausibility comparison with a further position signal, with a power restriction so that considerable impairments to the availability of the motor vehicle may occur. Safety monitoring functions which detect such error conditions and initiate the corresponding measures are known, for example, from U.S. Pat. No. 4,603,675.